The present invention relates to thin film photoelectric conversion devices and particularly to reducing the cost for and improving the performance of the silicon-based thin film photoelectric conversion devices.
In recent years, there has been a vigorous development of a photoelectric conversion device employing a silicon thin film, particularly that containing crystalline silicon such as polycrystalline silicon and microcrystalline silicon. In this development a low-temperature process is employed to form a crystalline-silicon thin film of good quality on an inexpensive substrate, attempting to reduce the cost for and enhance the performance of the device, and this development is expected to be applied to not only solar batteries but optical sensors and other various photoelectric conversion devices. Incidentally, it should be noted that throughout the present specification, the terms xe2x80x9cpolycrystallinexe2x80x9d, xe2x80x9cmicrocrystallinexe2x80x9d and xe2x80x9ccrystallinexe2x80x9d are used not only for meaning a completely crystalline state but also for meaning a xe2x80x9cpartially amorphousxe2x80x9d state, as commonly used in the technical field of thin film photoelectric conversion devices.
When a photoelectric conversion layer is a thin film, it insufficiently absorbs light in a longer wavelength range in which it has a small light absorption coefficient and it will thus have a limited level of photoelectric conversion that is attributed to its small thickness. Particularly in case of a photoelectric conversion layer including crystalline silicon, it does not cause sufficient light absorption. As such, to more efficiently utilize light incident on a photoelectric conversion unit including a photoelectric conversion layer, a photoelectric conversion unit is provided on its back side with a highly reflective metal layer having an uneven surface (a textured surface) to scatter and reflect light back into the photoelectric conversion unit.
Furthermore, on the light incidence side there is also provided a transparent electrode having an uneven surface (a textured surface) to scatter light into the photoelectric conversion unit and also to irregularly reflect again the light reflected from the metal electrode. Photoelectric conversion devices including a transparent electrode having a surface textured as above are disclosed for example in Japanese Patent Laying-Open Nos. 59-61973 and 7-283432, in which it is mentioned that the photoelectric conversion efficiency can be improved.
A silicon-based photoelectric conversion unit, as typically used in a thin film polycrystalline silicon solar battery, includes a photoelectric conversion layer formed of a silicon-based thin film, and conductivity types layers sandwiching the conversion layer. The conductivity type layers are doped with impurities, which absorb light and thus reduce the light incident on the photoelectric conversion layer. To decrease the quantity of light absorbed by such impurities and increase the light incident on the photoelectric conversion layer, it is preferred to reduce the thickness of the conductivity type layers within their permissible reduction range.
Under the condition as above, the present inventors have found that when a front transparent electrode and a back electrode each have an uneven surface capable of providing a preferable irregular reflection to allow a photoelectric conversion layer to absorb more light, thin film conductivity type layers in the photoelectric conversion unit that are in contact with the electrodes can have mechanical and electrical defects and the obtained solar battery can disadvantageously have a reduced open-circuit voltage or be short-circuited, resulting in a reduced yield thereof.
To overcome such disadvantages of the prior art found by the present inventors, the present invention contemplates a silicon-based thin film photoelectric conversion device allowed to use an inexpensive substrate and fabricated merely through a low-temperature process, capable of utilizing light confinement to provide an enhanced photoelectric conversion characteristic while neither its open-circuit voltage nor its production yield are reduced.
The present inventors studied hard to overcome the disadvantages found as above and have found that when a silicon-based thin film photoelectric conversion device includes a photoelectric conversion unit having semiconductor junctions formed by semiconductor layers all provided through plasma chemical vapor deposition (plasma CVD) at low temperature, a back electrode or a front transparent electrode can have a surface closer to the photoelectric conversion unit having an uneven texture controlled in level and pitch to enhance the thin film photoelectric conversion device in performance, achieving a high open-circuit voltage and allowing the photoelectric conversion layer to absorb a larger quantity of light.
More specifically, in the present invention a silicon-based thin film photoelectric conversion device includes a substrate, a back electrode having a light reflecting metal film, at least one silicon-based photoelectric conversion unit, and a front transparent electrode, wherein at least one of the light reflecting metal film and the front transparent electrode has a surface closer to the silicon-based photoelectric conversion unit having an uneven texture with a level difference in a range of 0.01 to 2 xcexcm and a pitch larger than the level difference and no more than 25 times the level difference. It should be noted that a level difference on a surface means an average of differences in altitude between convex portions and concave portions and that a pitch means an average of distances between adjacent convex portions or adjacent concave portions.
In the present invention, a silicon-based thin film photoelectric conversion device may have a light reflecting metal film having a surface closer to the silicon-based photoelectric conversion unit having an uneven texture with a level difference of 0.01 to 2 xcexcm and a pitch larger than the level difference and no more than 25 times the level difference.
Furthermore, a silicon-based thin film photoelectric conversion device may have a front transparent electrode having a surface closer to the silicon-based photoelectric conversion unit having an uneven texture with a level difference of 0.01 to 2 xcexcm and a pitch larger than the level difference and no more than 25 times the level difference.
These solar batteries may be a silicon-based thin film photoelectric conversion device having a light reflecting metal film and a front transparent electrode each having a surface closer to the silicon-based photoelectric conversion unit having an uneven texture with a level difference of 0.01 to 2 xcexcm and a pitch larger than the level difference and no more than 25 times the level difference.
Preferably, the light reflecting metal film or the front transparent electrode has a surface closer to the silicon-based photoelectric conversion unit having an uneven texture substantially not including bent points at which slopes of curves are discontinuously changed.
Furthermore, preferably, the light reflecting metal film or the front, transparent electrode has a surface closer to the silicon-based photoelectric conversion unit having an uneven texture substantially free bent points at which slopes of curves are discontinuously changed.
In recent years, it has often been tried to form a photoelectric conversion device with a silicon-based photoelectric conversion unit deposited on a back electrode including a metal layer and a transparent conductive layer of oxide such as zinc oxide (ZnO), as disclosed for example in Japanese Patent Laying-Open Nos. 3-99477 and 7-263731; IEEE 1st World Conf. on Photovoltaic Energy Conversion, p. 405 (1994); and Applied Physics Letters, Vol. 70, p.2975 (1997). The transparent conductive oxide layer interposed between the back electrode""s metal layer and the silicon-based photoelectric conversion unit can alleviate a thermal distortion attributed to the difference between their thermal expansion coefficients and also prevent the metal atoms from diffusing into and mixing with the silicon-based photoelectric conversion unit. Thus, the obtained photoelectric conversion device can be improved in yield and reliability as well as in photosensitivity and hence photoelectric conversion characteristics. In the present invention also, a transparent conductive oxide film may be provided between the light reflecting metal film and the silicon-based photoelectric conversion unit.
The present invention can remarkably effectively operate when at least one of photoelectric conversion units includes a layer of a first conductivity type, a crystalline silicon-based photoelectric conversion layer, and a layer of an opposite conductivity type.
In the present invention, preferably the photoelectric conversion device includes a metal film having a high reflectance reflecting no less than 95% of a light having a wavelength in a range of 500 to 1200 nm.
More specifically, the metal layer is preferably formed of one selected from the group of Ag, Au, Al, Cu and Pt or an alloy containing the same.
If a transparent, conductive oxide film is interposed between the light reflecting metal film and the silicon-based photoelectric conversion unit, it is preferable that the metal film""s interface with the transparent, conductive oxide film be formed of one selected from the group of Ag, Au, Al, Cu and Pt or an alloy containing the same.
If a crystalline silicon-based photoelectric conversion layer is used, preferably it is formed at a substrate temperature of at most 400xc2x0 C. and it has a crystallized volume fraction of at least 80%, a hydrogen content in a range of 1 to 30 at. %, a thickness of 0.5 to 20 xcexcm, a preferential crystal orientation plane of (110) parallel to its film surface, and an x-ray diffraction intensity ratio of no more than 0.2 as a ratio of a (111) diffraction peak to (220) diffraction peak ratio.
Furthermore, in the present invention a photoelectric conversion device may include a photoelectric conversion unit including a crystalline silicon-based photoelectric conversion layer plus at least one of photoelectric conversion units including an amorphous silicon-based photoelectric conversion layer and stacked in tandem.
In the present invention, a silicon-based photoelectric conversion device may have a back electrode having a light reflecting metal film, at least one silicon-based photoelectric conversion unit, and a front transparent electrode stacked in this order on a substrate.